With enhancement of the attentions paid to the environmental problem, efforts are being made in an attempt to decrease the weight of the part by increasing the strength of the part and by decreasing the thickness of the part. Further, with expansion of the field to which a high strength steel sheet is applied, the press forming tends to be employed widely for performing a complex process even in the case of handling a high strength steel sheet, with the result that required is a material having a high strength and, at the same, excellent in the workability.
Particularly, in the field of the automobile, the high strength steel sheet is required to exhibit various properties in addition to the balance between the strength and the stretch flange-ability. To be more specific, required are (1) a high yield ratio (YS/TS>0.7) in view of the safety in the event of a car crash, (2) an excellent balance between the strength and the uniform elongation (TS×U·EL>12,000) in view of the bulging properties, and (3) a good plating capability in view of the durability of the part (in general, Si<0.5% is one of the absolutely required conditions). Particularly, concerning the uniform elongation, i.e., requirement (2) given above, an improvement in the uniform elongation is a very important factor nowadays because the ductility until the starting of the necking after the yield point has come to be required in accordance with the complex shaping of the part and the shortening of the press forming time, which are required nowadays. However, it is very difficult for the conventional technology to satisfy simultaneously all the requirements (1) to (3) given above.
It was customary in the past to use a high strength steel sheet for the manufacture of a structural part and, thus, the stretch flangeability has been evaluated as more important than the bulging properties. Therefore, many methods have been proposed to date for satisfying the requirements for both the high strength and the high stretch flangeability. For example, proposed in each of JP-A-7-11382 and JP-A-6-200351 identified hereinafter is a steel sheet exhibiting an excellent hole expanding ratio in spite of a high strength not lower than 700 MPa. Specifically, it is proposed in patent document 1 that TiC or NbC is precipitated in the acicular ferrite structure so as to obtain a steel sheet excellent in the hole expanding ratio. On the other hand, it is proposed in JP-A-6-200351 that, in order to increase the hole expanding ratio of the steel sheet, at least 85% of the structure of the steel sheet is formed of a polygonal ferrite, that TiC is precipitated, and that Mo is dissolved. JP-A-7-11382 and JP-A-6-200351 also propose the methods of manufacturing the particular steel sheets. However, where TiC or NbC is utilized for precipitation strengthening as in the patent documents quoted above, it is unavoidable for the precipitate to be enlarged and coarsened, leading to a lowered strength. It is also difficult to secure a sufficient stretch flangeability because the enlarged and coarsened precipitates provide the starting points and the propagating route of the cracking.
In order to overcome the problems pointed out above, proposed in JP-A-2004-143518 referred to hereinafter is a steel sheet containing ferrite as a main phase and having V carbonitride, which has an average carbide diameter not larger than 50 nm, precipitated within the ferrite grains. It is taught that the steel of the particular structure permits improving the total elongation, the hole expanding ratio and the fatigue resistance. However, the structure obtained by this method consists mainly of ferrite and pearlite and is not intended to utilize the retained austenite and martensite (It is taught that it is highly desirable for the amount of the second phase to be 0%). It is not reasonable to state that the steel sheet proposed in patent document 3 is satisfactory in the balance between the strength and the uniform elongation. On the other hand, a steel sheet having a high YS/TS ratio, a good stretch flanging property, and a satisfactory plating property and a method of manufacturing the particular steel are disclosed in each of JP-A-2002-322539, JP-A-2002-322540, JP-A-2002-322541, JP-A-2002-322543, JP-A-2003-89848, JP-A-2003-138343 and JP-A-2003-138344 referred to hereinafter. It is taught that the steel sheet exhibiting the excellent properties can be obtained by the construction that the structure is formed of ferrite and the ferrite structure is reinforced by superfine precipitates containing Ti and Mo and having an average precipitate diameter not larger than 10 nm. The method proposed in these patent documents is highly effective in respect of requirement (1) referred to previously. However, the particular method is incapable of obtaining not only a ferrite single phase structure but also a good balance between the strength and the uniform elongation.
Various methods utilizing the retained austenite (retained γ) are proposed as a measure for improving the balance between the strength and the uniform elongation or between the strength and the entire elongation (EL). For example, a steel sheet excellent in the balance between the strength and the entire elongation and a method of manufacturing the particular steel sheet are disclosed in JP-A-2000-336455 referred to herein later. It is taught that the steel sheet has a composition containing 0.5 to 20 wt % of Si and 0.005 to 0.3 wt % of Ti, that the steel sheet contains ferrite having an average grain diameter smaller than 2.5 μm as a main component, and that the steel sheet has a structure containing bainite having an average grain diameter not larger than 5 μm and at least 5% of the retained γ. However, since the steel sheet is strengthened mainly in this prior art by grain refinement, it is difficult to obtain the requirement of YS/TS>0.7. It is also difficult to obtain the strength not lower than 780 MPa.
Disclosed in each of JP-A-4-228538 and JP-A-2003-321738 referred to hereinafter are a steel sheet having a strength not lower than 780 MPa and an excellent balance between the strength and the entire elongation and a method of manufacturing the particular steel sheet. It is disclosed in JP-A-4-228538 that the ratio of the polygonal ferrite space factor rate to the average grain diameter of the polygonal ferrite is set at 7 or more, and that Si is added in a large amount so as to obtain the steel sheet noted above. On the other hand, JP-A-2003-321738 teaches that the ferrite in the retained γ steel having Si added thereto in an amount of 0.5 wt % or more is reinforced by fine precipitates containing Ti and Mo so as to obtain the steel sheet noted above. In each of these methods, however, required is Si in an amount of 0.5 wt % or more so as to deteriorate the surface properties and to lower the plating capability of the steel sheet.
As a measure for obtaining a retained γ steel without adding a large amount of Si, disclosed in, for example, JP-A-6-264183 referred to hereinafter is a steel sheet excellent in the balance between the strength and the entire elongation. It is taught that the steel sheet contains 0.8 to 2.5 wt % of Sol. Al and that a fine polygonal ferrite containing at least 5% by volume of retained γ constitutes the main phase of the steel sheet. JP-A-6-264183 also discloses a method of manufacturing the particular steel sheet. In this prior art, a fine polygonal ferrite is used as the main phase of the steel sheet in order to improve the hole expanding ratio. It should be noted in this connection that the fine polygonal ferrite is solid-solution-strengthened by Si alone, or is precipitation-strengthened by TiC or NbC, with the result that the precipitates are enlarged and coarsened in the re-heating stage for applying a molten zinc plating to the surface of the steel sheet so as to give rise to the difficulty that the crystal grains are enlarged and coarsened so as to lower the strength and the hole expanding ratio. In addition, in order to obtain a fine polygonal ferrite, it is necessary to heat the steel sheet between rolls of at least two rear stage stands of a finish rolling mill in a temperature region of Ar3−50° C. to Ar3+100° C. with the total rolling reduction in this temperature region set at 30% or more. It is possible to supply current directly to the roll for heating the roll in order to heat the steel sheet between rolls of the finish rolling mill. In this method, however, special facilities are required. In addition, such a large power as 1,500 kVA is required, leaving room for further improvement in view of the energy saving.